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  • ‘Wasteful’ galaxies launch heavy elements into surrounding halos and deep space

    Spiral galaxies like the Milky Way are shown in the centre, and are the only objects visible to our eyes. However, much more lies beneath, including hot gas in the circumgalactic medium shown as red, orange and white on the left and dark matter structures shown as green on the right. Both the hot circumgalactic medium and dark matter component far outweigh the galaxies, but are invisible to our eyes. Credit: Adrien Thob, Liverpool John Moores University

    Galaxies “waste” large amounts of heavy elements generated by star formation by ejecting them up to a million light years away into their surrounding halos and deep space, according to a new study involving the Astrophysics Research Institute.

    The research, which was recently published online in the Monthly Notices of the Royal Astronomical Society, shows that more oxygen, carbon and iron atoms exist in the sprawling, gaseous halos outside of galaxies than exist within the galaxies themselves, leaving the galaxies with fewer raw materials needed to build stars, planets, and life itself.

    “Previously, we thought that these heavier elements would be recycled in to future generations of stars and contribute to forming planetary systems and providing the building blocks of life,” said Benjamin Oppenheimer, a research associate in the Center for Astrophysics & Space Astronomy (CASA) at CU-Boulder and lead author of the study. “As it turns out, galaxies aren’t very good at recycling.”

    The near-invisible reservoir of gas that surrounds a galaxy, known as the circumgalactic medium (CGM), is thought to play a central role in cycling elements in and out of the galaxy, but the exact mechanisms of this relationship remain elusive. A typical galaxy ranges in size from 30,000 to 100,000 light years while the CGM can span up to a million light years.

    The researchers used data from the Cosmic Origin Spectrograph (COS), an advanced spectrograph installed on NASA’s Hubble Space Telescope, which uses ultraviolet spectroscopy to study the evolution of the universe.

    Spiral galaxies like the Milky Way actively form stars and have a blueish colour while elliptical galaxies have little star formation and appear red. Both types of galaxies contain tens to hundreds of billions of stars that create heavy elements.

    After running a series of simulations, the researchers found that the CGM of both types of galaxies contained more than half of a galaxy’s heavier elements, suggesting that galaxies are not as efficient at retaining their raw materials as previously thought.

    “The remarkable similarity of the galaxies in our simulations to those targeted by astronomers using COS enables us to interpret observations with greater confidence,” said Robert Crain, a Royal Society University Research Fellow at the Astrophysics Research Institute of Liverpool John Moores University and a co-author of the study.

    The new simulations also explain the puzzling COS observation that there appears to be less oxygen around elliptical than spiral galaxies.

    "The CGM of the elliptical galaxies is hotter," said Joop Schaye, a professor at Leiden University in the Netherlands and a co-author of the study. "The high temperatures, topping over one million degrees Kelvin, reduce the fraction of the oxygen that is five times ionized, which is the ion observed by COS."

    By contrast, the temperature of the CGM gas in spiral galaxies is 300,000 degrees Kelvin, or around fifty times hotter than the surface of the Sun.

    “It takes massive amounts of energy from exploding supernovae and supermassive black holes to launch all these heavy elements into the CGM,” said Oppenheimer. “This is a violent and long-lasting process that can take over 10 billion years, which means that in a galaxy like the Milky Way, this highly ionized oxygen we’re observing has been there since before the Sun was born.”